I have replaced the fan swith limit switch and gas valuve. But the furnace comes on and after 30-45 seccs it shuts off the pilot, main gass and fan never comes on.. when the fan should come on it clicks and shuts down every thing. Any help. My suspicion is the wiring is not correct. ( th vs tr on gas valve and the limit is a spdt switch so I'm not sure about the limit wiring either


Thanks in advance

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Originally Posted by jimgott

i have replaced the fan swith limit switch and gas valuve. but the furnace comes on and after 30-45 seccs it shuts off the pilot, main gass and fan never comes on.. when the fan should come on it clicks and shuts down every thing. any help. My suspicion is the wiring is not correct. ( th vs tr on gas valve and the limit is a spdt switch so im not sure about the limit wiring either


thanks in advance

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Hi, I imagine it is not on long enough for heat to turn the blower on. I had one once that did the same thing. What was happening was when the burner came on and air started to flow to the flame it would blow out the pilot light and then the thermocouple would shut the whole thing down. What solved the problem was to bend a piece of tin where the air entered by the burner to change the air flow. Solved the problem. If a panel is missing or air is leaking in where it should not it could change the dynamics of the air feeding the system. Just an experience. This one fooled one HVAC repair man into replaceing the control valve but a diff. repair man figured it out.

I need more information. Age, brand, efficiency could really help me answer this one. There is something about Hannibals answer I'm just not sure of.

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Originally Posted by Hannibal

Hi, I imagine it is not on long enough for heat to turn the blower on. I had one once that did the same thing. What was happening was when the burner came on and air started to flow to the flame it would blow out the pilot light and then the thermocouple would shut the whole thing down. What solved the problem was to bend a piece of tin where the air entered by the burner to change the air flow. Solved the problem. If a panel is missing or air is leaking in where it should not it could change the dynamics of the air feeding the system. Just an experience. This one fooled one HVAC repair man into replaceing the control valve but a diff. repair man figured it out.

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Was the primary air disturbing the pilot? This should never happen. Or was it the draft inducer fan that caused the flame to lift of the thermocouple?

Quote:

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Originally Posted by jimgott

i have replaced the fan swith limit switch and gas valuve. but the furnace comes on and after 30-45 seccs it shuts off the pilot, main gass and fan never comes on.. when the fan should come on it clicks and shuts down every thing. any help. My suspicion is the wiring is not correct. ( th vs tr on gas valve and the limit is a spdt switch so im not sure about the limit wiring either


thanks in advance

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Are there 2 wires that come into the valve right below the thermocouple lock nut?

Hi, I am not an HVAC person. This is just what happened on a furnace I once had in a house I was renting to a HVAC person. He tried to fix it, said it needed a new regualator. So I had a friend who is a HVAC repairman and he sold me a new regulator/valve and the renter and I put it on. Still the prob. Remained. Then my friend the HVAC repairman came over and actually figured it out. The furnace was an older type and had no combustion motor. The primary air would I guess lift the pilot far enough off the thermocouple to shut it down. It is prob. A very rare instance.